Getting Wired: Spark Plug Wire 101

Spark Plug Wires. Important, yet un-sexy. There’s not many technical articles about spark plug wires, and for years we’ve been relegated to finding whatever information that was out there in the form of advertisements from manufacturers or on the back of the spark plug wire box. We’re going to suppress the interference and get down to the truth about plug wires: whether you’re picking a set of wires for your daily driver, pimped out ghetto cruiser or balls-out race car, you’ll want to get the right info… and you’ve found the right place.

Spark Plug Wire 101:

A spark plug wire is an electrical conductor. Period.

It’s job is to act as a conductor of moveable electricity or more specifically, a moveable electrical charge. Plug wires are unique in the aspect that they don’t have any mechanical moving parts, but can breakdown or interfere with nearby delicate electronic components in the engine bay. The demands on plug wires today are greater than they have ever been before. Higher voltage current, higher temperatures under the hood and technically advanced ignition systems have created an atmosphere where selecting the right plug wire can optimize engine performance or degrade the engine’s output.

The key word here is “optimize” engine performance. If you’re looking to add horsepower by buying a new set of plug wires, then you’ve been led down the wrong path. To get the real story on plug wires, we visited with the experts from MSD, ACCEL, and Granatelli Motorsports to get us through the smoke and mirrors and find out the real deal on spark plug wires.

Plug Wire Technology Advancements

Back in the old days, consumers were offered two types of plug wires.

An OEM replacement wire that consisted of a carbon core.

A race plug wire that consisted of wound wire strands that were commonly referred to as Solid Core wires.

The true difference between these sets of plug wires lay in the resistance of the wires. The carbon core wire had a much higher resistance rating and limited the amount of electrical charge that was passed to the spark plugs. Because of this, resistance rating became the standard when comparing spark plug wires.

Low resistance suppression wire cut so that the layers are visible.

Suppression spark plug wires came on the scene next.

With the advances made in electronic ignitions, higher electrical charges were being moved through the spark plug wires. As electricity moves through a conductor, like a spark plug wire, there is an electromagnetic field created around the conductor.

According to Joe Pando, Service Manager at MSD Ignition, Electromagnetic Interference (EMI) began to be a problem. It was picked up by antennas and heard through car radios. When the automobiles computer systems became more pronounced with monitoring systems for every aspect of operation, EMI was guilty of sending random signals that were picked up by these sensors.

“The reverse is also true,” explained Pando. “These Engine and Environmental control units were moving electricity and creating their own electromagnetic fields which often were picked up by the spark plug wires causing a faulty signal. EMI signal that interfere with sensors that relay information back to engine management computers range from intermittent misses to severe loss of power.”

These problems are can be exacerbated by using high output ignition systems. To combat the effects of the electromagnetic interference, a fine wire of steel alloy was wrapped around the core of the spark plug wire to suppress the EMI and Radio Frequency Interference (RFI). Most spark plug wire manufacturers used the alloy Monel as the wire wrap around the core. Monel is an alloy that uses a blend of metals including stainless steel, with small percentages of copper and nickel.

Here’s where the lines started to get blurry. Manufacturers of Suppression wires have spent a great deal of research and development finding the right balance in number of wire coils around the the conductor core. More coils reduces RFI and EMI but raise the electrical resistance in the wire. Because resistance measurement had become the standard unit of measurement when selecting solid core and carbon core plug wires, that standard has carried over in marketing the newer type wires.

Understanding spark plug wire resistance ranges and the application for each type of range is critical when purchasing plug wires.

Why does Resistance Matter?

Let’s take a look at the stock OEM plug wires as a reference point for a conservative type plug wire. OEM type plug wires are usually a carbon fiber that helps minimize RF noise. The drawback to using carbon fibers is a higher resistance in the plug wire which reduces the amount of electrical charge that can be delivered to the spark plug.

Solid core plug wires on the other hand have a very low resistance which can cause extreme levels of RF interference and random signals to other electrical components and is a good example of a pure race type plug wire where limited electrical components are used.

Tim Musiek of the Prestolite Performance, the parent company of Mallory, Accel and Prestolite wire, gave us a pretty safe range for selecting plug wires based on resistance. According to Tim, “My recommendation for a bone stock daily driver is to not go below 500 ohms per foot wire resistance. For a circle cars or door slammers, you can go down as far as you are comfortable. The only time I would go with a solid wire core is with an ignition system that has a magneto or points. Sprint cars would be a good example.”

There is an interesting breakthrough in solid core wires from Granatelli Motor Sports that offers the extremely low wire resistance and durability of solid core wires, yet according to GMS, suppresses the EMI and EFI “cross chatter” between electrical components.

J.R. Granatelli calls his patented process “the pepto bismol for the engine’s heartburn.” Granatelli explained by using a “Suppression Ring” on the outside of the wire, the Granatelli solid core wires “offer a near zero ohm resistance to the plug from the coil and still prevent the top end break up” that used to plague solid core wires in vehicles with multiple engine reference sensors.

Both Joe Pando of MSD and Tim Musiek agree that spark plugs can only use the electrical energy that they were designed for and for the purpose that they were designed for.

Joe explained that the “plugs use only the energy it takes to cross the electrode gap in the atmosphere of the combustion chamber with atomized fuel under compression”. The energy consumed at the spark plug gap won’t be any more than what it takes to jump the gap, similar to a 50 watt bulb in a lamp that won’t get brighter or use more energy even if it is used in a lamp that is wired to supply a 100 watt light bulb.

Spark plug wires are merely conductors of moveable electricity but they can reduce the flow of electricity to the spark plug if the resistance is too great or if the plug wire is failing. Joe went on to explain that plug wires were “Consumables” and they would get brittle and crack with age. Electrical charges can leak through those areas, taking the path of least resistance “resulting in a lazy spark”.

J.R. Granatelli’s take on electrical charge is a little different because of the nature of their solid core wires. Granatelli explained that a static ignition system will work one way when it is at idle, “but with forced induction, altitude changes or under and extreme load, the scenario changes completely. You want that extra voltage available to keep up with the changes in the cylinder atmosphere”. A simple case of supply meeting the demand.

Anatomy of a Spark Plug Wire

Plug wires are almost all constructed with very similar components consisting of a Center Core, a Conductive suppression layer, an Insulator, a fiberglass braid, and the outer silicone jacket.

The outer Silicone jacket is the most visible part of the spark plug wire and gets the most attention in the marketplace. Most wire manufacturers offer several different colors of outer jackets for that extra special “look” under the hood, however, the outer jacket serves a much more valuable purpose than simply looking good. Protection from heat, abrasion, oxidation from ozone exposure and deterioration from chemical exposure.

As with all the other layers in the spark plug wire, the outer jacket also adds strength to the wire. Both Tim Musiek of Accel and Joe Pando of MSD agree that the outer jackets on plug wires have significantly improved in the past couple of decades. Surface contamination and radiant heat are the two biggest problems for plug wires. Either of these conditions can cause a serious loss of power out of the wire to a ground when touching metal or a grounded surface.

MSD’s Pando jokingly offered us a tried and true method of checking your plug wires for shorts. “With the car running, put some water on your hands and run them along the length of the wire. That will make you appreciate good silicone jackets”.

Under the outer jacket is a layer of braided material, usually fiberglass, which adds strength and helps minimize EMI. Many mechanics remove spark plug wires from the distributor cap and spark plugs by pulling on the wire. Without this braided layer of fiberglass, the conductor core could easily be broken by pulling on the plug wire or severe engine vibration. The braided layer also helps retain the crimped terminals at the ends of the plug wire when the terminals are crimped correctly.

Accel’s Tim Musiek reminded us that “using the right size terminal for the size of the jacket and braided layer is extremely important. We (Accel) sell wires from 5mm diameter to 10.8mm. The terminal used should be sized to the wire. One size doesn’t fit all”.

The layer below the fiberglass braided layer is usually a very dense, heat resistant silicone insulation. Silicones have measured resistance levels of about 10 to 11 ohms per meter. Keeping in mind that our insulator is roughly 2.5mm thick, it is probable that some electrical charge can leak through the insulation if it is cracked or deteriorated. To minimize the chance for energy loss, EMI or EFI getting through to the outer layers, most manufacturers use a treated layer between the conductor/suppression layer and the insulation.

Joe Pando explained that MSD uses a proprietary coating on the center core that aids in minimizing the EMI. According to Pando, “the secret is in the sauce. We use the secret sauce to keep the electromagnetic interference and Radio Frequency interference down below normal levels”.

The heart of it all is the core. Typically the conductor core in plug wires have been carbon impregnated fibers, everything from nylon to kevlar type fibers have been used, to help transfer moveable electrical charges from the distributor to the spark plug. For very high performance vehicles, a solid core wire is used. The RFI and EMI are levels are unacceptable high for computer controlled vehicles however and the solid core wires are typically only used in very high performance, non-computer controlled vehicles.

Types of Spark Plug Wires:

Carbon Core “OEM” Wires

OEM wires work well with bone stock factory cars. For your typical grocery getter and ghetto cruiser, the OEM wires will work well getting spark to the spark plugs and won’t push the engine beyond factory specs. They require frequent replacement as the carbon core breaks down or cracks. Keep in mind that the factory treats plug wires as consumables and placed on a routine replacement cycle. Using a high powered ignition system will shorten the life of these plug wires.

Solid Core Wires

Solid core wires are usually made with a stainless steel or copper core, both of which conduct high electrical current very well. They also have no RFI suppression ability and that makes them a poor choice for electronic ignition systems. The frequencies broadcast into the atmosphere interfere with the electronic modules in the area. These should only be used on high performance cars that use magnetos or a points and condenser type ignition system. As with any rule, there are exceptions. The Granatelli Motor Sports solid core wires have proven to be very effective at suppressing the EMI and RFI emissions and still maintain the superior low resistance conductivity of solid core wires.

Low Resistance “Spiral” type WiresThe most popular plug wires for high performance street vehicles and race cars is the Spiral type wires. The construction of these wires is done by winding a very fine wire around the core. This wire is typically an alloy of copper, nickel or tin and usually a combination of the three metals. This winding serves to lower the RFI and EMI and still have a lower resistance in the wire. Spiral type wires should be considered when using any high performance ignition system or have upgrades to the engine that raises the cylinder pressure or add more fuel to the combustion. Our experts from MSD and Accel also agree that the higher the cylinder pressure and the more fuel there is, the higher the voltage needs to be to start a spark in the combustion chamber.

Cross-sections of some popular brands of low resistance suppression type plug wires.

Wrapping It All Up

Picking the right plug wire for your application is fairly easy based on what we learned from our experts at MSD, Accel and Granatelli Motor Sports. If you are running a bone stock car on the street as a daily commuter then the OEM carbon core wires are probably fine. Any upgrades in performance, especially in the fuel or ignition system would require Spiral type wires. Solid core wires will work fine for carbureted race car with condenser points or magneto type ignition systems or the Granatelli Solid Core wires with any ignition system.

And size matters. Tim Muskeik of Accel explained that “some sprint car teams use our 5mm spark plug wires to help save on weight and EMI is not an issue”. Most every other type of street and performance car will use an 8 – 8.5mm diameter plug wires with no problems.

Joe Pando of MSD summed up spark plug wire performance with the warning about low suppression Spiral type wires; “spark plug wires required periodic changing. If you want to keep the performance of your engine at the optimal level, replace the plug wires as often as you replace the spark plugs”.

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About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.